The invention relates to a method of heat treatment of products advanced through a chamber by circulating through the chamber hot, humid air that is exhausted and supplied with humidity and heat and then blown back to the chamber, and an apparatus for use in the method.
The method and the apparatus according to the invention are generally applicable and may thus for instance be used for thawing out frozen products but have primarily been developed and tested in connection with the scalding of carcasses, in particular pig carcasses, and the invention will therefore be described in the following particularly in connection with such application.
It is known in so-called suspension steam scalding to scald pig carcasses in a chamber by treatment with hot, humid air or air containing particles of water. The pig carcasses are then scalded either by the condensation of the steam on the rind surface or by hot particles of water from the circulating air.
NO patent No. 105 798 discloses an apparatus for the treatment of carcases in a chamber with a vertically rising flow of steam by exhaustion of vapour from the upper end and blowback of the steam to the lower end. Humidity is supplied as vapour by means of an evaporator at the lower end of the chamber.
DK patent No. 116 190 deals with an apparatus for thermally treating products, in particular for scalding carcases, in which steam is applied to the circulating air, following which it is guided past refrigeration units causing the humidity of the air to condensate as a fog of floating droplets. The products are then thermally treated with a fog of hot particles of water. The apparatus is extremely energy consuming because large amounts of heat get lost through the refrigeration units.
In another practised apparatus the consumption of vapour is reduced by replacing the refrigeration units by heating units which together with the supply of aqueous vapour maintain a well defined temperature and humidity of the air in a chamber, e.g. 85 to 90% of relative humidity. The control of the heating units and the supply of vapour are effected by means of a humidity sensor and a temperature detector, but at varying loads of the chamber it has turned out to be difficult to obtain satisfactory joint control of temperature and humidity. An example of a process utilizing such an apparatus is disclosed in DK patent application No. 5006/84 dealing with a method of scalding carcases by directing a hot, humid jet of air having a relative humidity of no less than about 80% and a temperature of about 62.degree. C. towards every part of the suspended carcases.
DK patent application No. 1643/85 relates to an apparatus for scalding suspended carcases in which heat and humidity are imparted to the circulating air by means of a combination of water atomizers and burners discharging directly in the duct for blowback of the circulating air to the chamber. A problem involved by this method is the presence of exhaust gas in the treatment air.
With a view to thawing out frozen products use is nowadays made of chambers with injection of hot air with controlled humidity.
In this respect the control of temperature and time according to a fixed program for the actual product is generally practised. It is very important that the temperature is controlled very strictly and that the air humidity is kept above a determined level, since too dry air, e.g. when thawing out frozen products, causes such products to become dry on their surface, thereby lowering their quality and value, and since a too quick thawing out implies a too heavy loss of drippage, following which the products also decline.
A general problem of the known methods and apparatuses is unstableness at fluctuating loads and other heat/cold stresses from outside, thereby hampering an exact, controlled heat treatment.
It is the object of the present invention to provide an inexpensive and effective method of heat treatment capable of eliminating unstableness as regards temperature and humidity at varying load or other heat/cold stresses.